Recent advances in portable electronic devices have been accompanied by increased interest in the electrochemical cells that supply power to these devices. Of particular interest are secondary energy cells, i.e., batteries or cells in which electrical energy can be repeatedly drained and recharged. Lithium-metal and lithium-ion secondary cells have very high electrical energy storage capacity, or energy density, and are the subject of vigorous development efforts. Typically, lithium secondary cells include a porous dielectric separator, or diaphragm, interposed between the electrodes of the cell and a liquid electrolyte to provide ionic conductivity. For convenience, "lithium secondary cells" or "lithium cells", will be used herein to include both lithium-metal and lithium-ion type batteries and secondary cells.
Also known are lithium secondary cells in which the electrolyte is a solid polymer electrolyte or a polymeric gel electrolyte. An example of a solid polymer electrolyte is a complex type electrolyte in which a metal salt, such as a lithium salt, and a polymer complex, such as a polyethylene oxide complex, each providing ligands, or complexing agents, are combined together. Ion transport is effected by movement of salt ions between ligands of the polymer complex, and is limited by the mobility of the side chains having ligands of the polymer complex. Such a solid polymer electrolyte has good mechanical strength, however, at room temperature, side chain mobility is quite low and ionic conductivity is limited to about 0.1 mS/cm.
In contrast to the solid polymer electrolyte described above is a polymeric gel electrolyte in which a liquid solvent/metal salt system is combined with a polymeric component to provide solid-like material properties. With this type of electrolyte ionic conductivity as high as about 1 mS/cm can be achieved by increasing the ion-conductive component, however, there is a concomitant decrease in tensile strength of the material. In such materials, when the ionic conductivity is high the tensile strength is only about 0.5 MPa, even when the polymeric component is crosslinked (see, for example, D. R. McFarlane, et al., Electrochimica Acta, Vol. 40, p.2131 1995!). Accordingly, such materials require mechanical support in order to be useful as a separator or diaphragm in a battery or electrochemical cell.
Porous membranes of polyolefins, such as polyethylene and polypropylene, impregnated with polymer gel electrolyte are known for use as separators in lithium secondary cells. For example, a lithium secondary battery having a separator comprising a microporous polyethylene membrane impregnated with a polymer gel electrolyte is disclosed in U.S. Pat. No. 5,597,659 (to Morigaki, et al.). In another example, a solution consisting of ethylene carbonate, propylene carbonate, tetraethylene glycol dimethyl ether (tetraglyme), LiAsF6, tetraethylene glycol diacrylate, and a small amount of a photopolymerization initiator is impregnated into porous polyethylene and polypropylene membranes and polymerized to form a solid electrolyte (K. M. Abraham, et al., Journal of the Electrochemical Society, Vol. 142, p. 683 (1995). Such membrane-supported solid polymer electrolytes and polymer gel electrolytes for separators in lithium cells represent a significant improvement in the state of the art, however, the gains in strength were made at the expense of ion conductivity, which was reduced by more than one order of magnitude compared to the polymer electrolytes when unsupported.
Properties which must be considered for selection of a support material for a solid polymer electrolyte or polymer gel electrolyte for use as a separator between electrodes in an electrochemical cell, in particular for a high energy cell such as a lithium secondary cell, include: chemical compatibility with the electrode materials and electrolytes; strength, sufficient to withstand the rigors of manufacturing and use; thickness, thin materials being desirable to minimize ion transport distance and maximize transport rate; and porosity, a combination of pore size, pore volume, and structure, which should be optimized to provide for introduction and retention of the electrolyte and to maximize ion conductivity between the electrodes. However, because many of these properties interact in opposition to each other, optimization of each property may be mutually excluded and compromises must be accepted. For example, it is desirable that a support material have a high pore volume and high strength, however, other characteristics being kept equal, as porosity increases strength decreases, which may lead to a tradeoff between strength required and pore volume desired.
Polyethylene and polypropylene have been used as separators, or as separators supporting solid polymer electrolytes and polymer gel electrolytes, between electrodes in lithium secondary cells due to their chemical compatibility with the electrode materials and electrolytes used in the cells. However, in such materials, porous membranes having the combination of strength, thickness, and porosity characteristics desired for greatest effectiveness are unavailable. When suitable thinness and porosity is obtainable, for example, about 1 micrometer thick and pore volume greater than 70 percent, the pore size available is so small as to make it difficult to impregnate the membrane with the electrolyte; and furthermore, the tensile strength may be quite low generally, or may be anisotropic, having a higher tensile strength in one planar direction than the tensile strength in the orthogonal planar direction which is relatively much lower. Low general tensile strength, or anisotropic tensile strength, can lead to handling and manufacturing difficulties, or may lower puncture and tear resistance of the porous membrane.
It is seen, then, that the effectiveness of membrane-supported solid polymer electrolytes and polymer gel electrolytes in lithium secondary cells can be highly influenced by the nature of the membrane support. It is the purpose of this invention to provide a membrane-supported polymer electrolyte separator for an electrochemical secondary cell which combines high strength, at least 10 MPa in at least two orthogonal directions, and high ion conductivity of at least 1 mS/cm, which heretofore has been unavailable.